1. Field of the Invention
The present invention relates to a method for producing a microporous membrane on whose surface an electron beam radiation crosslinked polymer is fixed, comprising the steps of providing a microporous starting membrane, impregnating the membrane with an impregnating solution comprising a solvent and also a polymer dissolved or dispersed therein to provide an impregnated membrane, and irradiating the impregnated membrane with electron beam radiation to provide a microporous membrane on whose surface the electron beam radiation crosslinked polymer is fixed, and also to a microporous membrane produced by this method.
2. Description of the Related Art
It is usually desirable, for the practical applications of membrane filters, that the filter material be mechanically, thermally and chemically stable and insoluble in common (organic) solvents. This is why polymers are frequently used as filter materials even though their surface properties, such as wettability with water or unspecific binding (of proteins for example), are not in line with the requirements in the later use. To improve the surface properties of membrane filters produced using these polymers, different methods had been developed over recent decades to permanently modify the membrane surfaces. Owing to the need for the modification to be permanent, simply impregnating the membrane with a solution, say, of a hydrophilic polymer (to hydrophilicize a hydrophobic polymeric membrane) is out of the question, since this hydrophilic polymer is readily washed back off the membrane. U.S. Pat. No. 4,698,388 discloses polymeric materials coatable with a block copolymer which can be synthesized, in the presence of a polymeric peroxide, from two varieties of vinyl monomers where the polymer of one of the two varieties of vinyl monomers is homogeneously dispersible in the polymeric materials and the other variety forms hydrophilic homopolymers. The durability of the hydrophilic modification, for example characterized by the antistatic properties or the electrical surface resistivity, is only tested under mild conditions, i.e., by rinsing with city water at room temperature for two hours, nothing else is said about the permanence of the hydrophilic modification on the polymeric material. US-A-2003/148017 discloses the modification of hydrophobic dialysis membranes based on poly(ether) sulfone by adsorption of copolymers having hydrophobic polypropylene oxide (PPO) segments and having hydrophilic polyethylene oxide (PEO) segments, the washoff of which from the membrane into the dialyzate solution is unconcerning in hemodialysis, but undesirable in sterile filtration.
To permanently modify membrane filters made of polymers having hydrophobic surfaces a priori unsuitable for certain filtration processes, U.S. Pat. No. 4,618,533 proposes impregnating porous hydrophobic crude membranes based on poly(ether) sulfone or polyvinylidene fluoride with a solution of a monomer (hydroxyalkyl or aminoalkyl acrylates) and of an initiator, and then to initiate the polymerization of the monomer through energy supply, for example by heating (thermal initiation) or UV irradiation (using a photoinitiator). The polymerization produces long-chain crosslinked chains of polymer in the interior of the porous matrix which are not washable off the membrane and are also partly grafted onto the membrane, effecting a permanent modification. Owing to the toxicity and carcinogenic effect of the monomers used (acrylates, methacrylates and other vinylic monomers), however, the difficulty with these processes is to ensure that no unconverted monomers remaining in the membrane after the modification be released in later use, for example into the filtrate. Reducing the level of extractables to what is permitted generally requires a high, cost- and time-intensive rinsing effort after the modification. In addition, the modification process requires toxic and carcinogenic compounds to be handled in the course of the manufacturing process and for the waste and residue materials generated in the course of that process to be disposed of. Hence using these compounds appears to be dubious not just from cost, environmental and health viewpoints but also, more particularly, against the background of ever stricter environmental regulations (“Reach”).
One alternative to using the frequently toxicologically concerning monomers mentioned is to use so-called prepolymers, i.e., polymers containing reacting functional groups. U.S. Pat. No. 5,629,084 discloses impregnating base membranes of polyvinylidene difluoride, polytetrafluoroetheane, polyethene and polypropylene with a solution of prepolymers using a free-radical initiator to initiate the polymerization. A crosslinker is not used, the crosslinking of the prepolymers based on polyvinyl alcohols, polyvinylamines, polyacrylic acids and polyvinylpyrrolidone and the grafting thereof to the membrane surface being induced through energy supply, specifically heat supply in the case of thermal initiators or UV irradiation in the case of photoinitiators. This process is disadvantageous in that it utilizes peroxide- and persulfate-based initiators in considerable proportions between 10 and 30 weight percent during production, since these or their degradation products (alkali metal sulfates, alkali metal phosphates and benzoic acid derivatives) have to be removed off the modified membrane material as waste materials by extraction after the modifying process. Furthermore, the production process is limited to thermal energy and UV radiation as radiation modes since electron beam radiation destroys the base membrane material and thus fails to provide any modification (cf. U.S. Pat. No. 5,629,084 column 3, lines 53 to 56). This finding from the prior art is supported by the fact that, in all exemplary embodiments, the modification of the starting membrane by crosslinking the prepolymer is only performed by heating or by UV irradiation, necessitating the use of, respectively, a free-radical initiator or a photoinitiator (cf. Examples 1-11 of U.S. Pat. No. 5,629,084).
One alternative to using initiators is to initiate the polymerization using electron beam radiation. For this, the membrane to be modified is impregnated with a solution containing reactive organic molecules, for example monomers derived from acrylic acid derivatives, and then exposed to electron beam radiation which initiates the reaction of the monomers. For modifying membranes, this process is described in U.S. Pat. No. 4,944,879, EP-B-1 390 087, EP-B-1 381 447 and U.S. Pat. No. 7,067,058. The impregnating solutions described frequently further contain crosslinkers, i.e., doubly or generally multiply functional monomers (cf. U.S. Pat. No. 4,944,879, EP-B-1 390 087, EP-B-1 381 447, WO-A-2005/077500) or mixtures of two or more monomers and crosslinkers (EP-B-1 390 087, EP-B-1 381 447 and U.S. Pat. No. 7,067,058). However, these processes likewise utilize toxic monomers such as acrylates, acrylamides and other vinylic monomers. Moreover, the crosslinkers are likewise toxic and/or caustic compounds. The problem of the extractable toxic constituents of the modified membrane which have to be removed in costly and inconvenient cleaning steps after membrane production is not solved in this way. Similarly, the variants of the thermally induced modification with a homopolymer and crosslinker (WO-A-2005/077500) or with an oligomer, with a monomer and optionally with a crosslinker utilize concerning low molecular weight organic compounds, while U.S. Pat. No. 6,039,872 discloses hydrophilic membranes obtainable from a hydrophobic poly(ether) sulfone membrane and a crosslinkable polyalkylene glycol diacrylate and also a hydrophilic monoacrylate. In addition, the processes described are complex and cost-intensive on account of the large number of different starting materials.
A further variant of this modification method of electron beam induced polymerization describes polymeric interlayers which are grafted onto the membrane surface, the polymeric interlayer being in turn grafted with a further modifying layer that endows the final membrane with hydrophilic properties. U.S. Pat. No. 6,509,098 and U.S. Pat. No. 6,616,982 disclose membranes and their methods of making, wherein the modification of hydrophobic base membranes is effected using an intermediary polymeric layer which can be generated from (meth)acrylic acid monomers by electron beam irradiation. This intermediary polymeric layer subsequently has applied to it, by electron beam induced grafting of polyethylene oxide, a hydrophilic, second polymeric layer. The intermediary polymeric layer is obtained from toxicologically concerning, monomeric acrylic acid derivatives, and this two-stage process is likewise comparatively costly and inconvenient.
The problem addressed by the present invention is therefore that of providing a microporous membrane that has a durable modification on its surface and does not contain any extractable toxic components, and also that of providing an environmentally friendly method for producing it without using toxicologically concerning starting materials and toxic process-initiating substances.